New cloaking surface throws an electromagnetic curveball

For centuries, humans have dreamed of fading into the background. Hunters have long wished to vanish into their surroundings, and these days, awkward moments at parties can evoke similar desires. But the ability to truly disappear is still only found in tales spun by writers and filmmakers, as cloaking has remained the stuff of fantasy and science fiction. Thanks to some pioneering researchers, however, cloaking has moved one step closer to reality.

Six scientists have built a sophisticated metamaterial that literally bends electromagnetic waves, according to a new paper published Thursday in the journal Science. Ruopeng Liu and Chunlin Li, researchers in David R. Smith’s lab at Duke University, along with three other colleagues, assembled more than 10,000 specially designed pieces to form a mat 20 inches long and four inches wide. When finished, the yellow pad sucked microwaves in and spit them out—with a curve.

To test their new invention, researchers first beamed microwaves at a flat, mirrored surface. The waves behaved as they should, bouncing off at a predictable angle. Next they shot it at a bump in a mirrored surface. The microwaves bounced and scattered, carefully obeying the laws of physics. Then the scientists laid their yellow mat over the bump. And the wave ignored the bump—or so it seemed. After reflecting off the curved surface, the radiation veered downward and continued along a flat surface-trajectory. The mat had cloaked the bump.

The results of the new study. Microwaves bounce off a flat surface in (A) while they both bounce and scatter off the curved surface in (B). With the new cloaking material, various frequencies of microwaves curve after they leave the curved surface (C through F), making it appear to sensors as though the surface were flat.Image courtesy AAAS/Science

Aside from starring in the Harry Potter series and countless Star Trek incarnations, cloaking has been a very serious and very active research area in the past few years. In May 2006, two scientists proposed active cloaking devices based on superlenses, but their contribution at the time was only theoretical. The reality of superlenses hasn't been as promising, but in October of that same year, Smith and his colleagues presented a cloaking breakthrough—the ability to cloak an object from a specific microwave frequency. More papers followed, and the science progressed rapidly. But then last December, a new theoretical study published gave researchers a harsh reality check—cloaking at multiple frequencies may very well be impossible, the authors said. As the number of cloaked frequencies increases, the efficacy of the device or material decreases. It’s a classic tradeoff, they implied, and one not likely to be overcome.

Liu and Li’s new research, though, seems to poke a giant hole in that last paper. Their new metamaterial masks not one tiny slice of the microwave spectrum, but a relatively large swath of it, from 13 to 16 gigahertz. Liu and Li built off the results of Smith’s 2006 paper to create the new cloak, but this time used more powerful algorithm to help them fabricate the metamaterial. The formula dictated where each of the over 10,000 pieces in the structure should be placed to achieve the desired effect.

"The difference between the original device and the latest model is like night and day," Smith said in a press release. While the earlier, more limited device took Smith and his team four months to build, the new, more capable cloak was ready in only nine days.

Smith compares the mat’s cloaking effect to a mirage. "You see what looks like water hovering over the road, but it is in reality a reflection from the sky," he said. "The mirage you see is cloaking the road below."

While you won’t be able to don a fancy blanket and duck out of work early any time soon, the new metamaterial proves that one surface can cloak many frequencies. Three gigahertz is certainly a far cry from the 350,000 GHz that make up the visible spectrum, but at least it’s a step in the right direction.